Parkinson's disease (PD) is a devastating movement disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra, where mechanisms of DA neuron loss are poorly understood. While PD affects approximately 1.5% of the North American population, available treatments only temporarily ameliorate PD symptoms and can not slow disease progression. The majority of PD cases are sporadic and environmental toxicants are linked to PD etiology. Microglia, the resident macrophage in the brain, are believed to contribute to the progressive nature of PD. Microglia are activated upon DA neuron injury to result in inflammation and damage to neighboring DA neurons (reactive microgliosis), but the mechanisms responsible are largely unknown. Here, we address the over-arching hypothesis that soluble neuron-injury factors are released upon environmental insult (MPP+/MPTP) to promote microglial activation, which drives further DA neurotoxicity, to result in a vicious, self-propelling cycle. This study is focused on u calpain, an intracellular calcium-dependant protease that is reported to be released extracellularly upon cortical neuron damage. Using a combined in vitro/in vivo approach, we will test the specific hypothesis that u calpain is a key soluble factor released upon DA neuron damage with MPP+/MPTP to activate microglia, which then potentiates additional DA neurotoxicity. The specific aims of this proposal are to: 1) determine the pro-inflammatory and neurotoxic characteristics of soluble factors released from DA neurons exposed to the direct neurotoxicant MPP+ (Mentored Phase); 2) characterize u calpain as a soluble neuron-injury factor contributing to reactive microgliosis (Independent Phase); 3) characterize the MAC1 receptor-mediated mechanism of u calpain-induced microglia activation and DA neurotoxicity (Independent Phase); 4) define the enhancing action of u calpain on progressive neurodegeneration, both in vitro and in an in vivo MPTP mouse model (Independent Phase). The proposed studies will reveal novel molecular signals that drive self-propelling neurodegeneration and identify therapeutic targets with the potential to slow PD progression. Additionally, this research will establish the groundwork for further studies into the mechanisms by which environmental factors contribute to reactive microgliosis, progressive neurotoxicity, and PD.